1. Field of the Invention
The present invention relates to an image pickup apparatus particularly with high resolution capability.
2. Description of Related Background Art
A video signal obtained by picking up an optical image of an object with an image pickup apparatus can be easily processed such as editing, trimming and the like. Recording/reproducing a video signal can also be easily performed by using an alterable memory. With a conventional image pickup apparatus, an optical image of an object is focussed onto a photo-conversion region of an image pickup element by a taking lens to be converted into electric image information. The electric image information is time sequentially outputted as a video signal. Various image tubes or solid state image pickup elements have been used for such conventional image pickup apparatus.
It is also known that various new television systems such as EDTV, HDTV and the like have been proposed for recent requirements of high quality and resolution of reproduced images.
In order to reproduce images with high quality and resolution, it is necessary for an image pickup apparatus to generate a video signal by which an image of high quality and resolution can be reproduced. However, an image pickup apparatus using an image tube has a limit in reducing the diameter of an electron beam, and also the target capacity thereof increases as its size becomes large, resulting in a poor resolution. Further, for high resolution moving images, the frequency band of a video signal becomes larger than several tens to hundreds MHz, thus posing a problem of poor S/N. The above problems have made it difficult to obtain reproduced images of high quality and resolution.
More specifically, in order to obtain a video signal by which an image of high quality and resolution can be reproduced, it is necessary to reduce the diameter of an electron beam or use a target of large area. However, there is a limit in reducing the diameter of an electron beam because of the performance of electron gun and the structure of focussing system. If the area of a target is made large using a large taking lens, the target capacitance becomes large so that the high frequency component of a video signal is degraded and hence the S/N thereof becomes considerably bad. Thus, it is impossible for an image pickup apparatus with an image tube to obtain a video signal for a reproduced image of high quality and resolution.
In the case of an image pickup apparatus with a solid state image pickup element, in order to obtain a video signal by which an image of high quality and resolution can be reproduced, it is necessary to use a solid state image pickup element having a large number of picture elements. However, with the solid image element having a large number of picture elements, the drive clock frequency becomes high (in the case of moving images, it is about several hundreds MHz) and the capacitance of the drive circuit becomes large as the number of picture elements increases. The clock frequency presently used for a solid state image pickup element is generally about 20 MHz so that such an image pickup apparatus cannot be used in practice.
As above, conventional image pickup apparatus cannot generate a video signal suitable for high resolution and quality of reproduced images.
The applicant company of this invention has proposed an image pickup apparatus capable of solving the above problems. According to such an image pickup apparatus, an optical image of an object to be image picked up is focussed with a taking lens onto to a photo-photo conversion element constructed of two transparent electrodes with at least a photo-conductive layer, dielectric mirror and photo-modulation layer interposed therebetween, and optical image information of the object is optically read from the photo-photo conversion element and photoelectrically converted into video signals with high resolution. FIG. 2 is a side elevational cross section illustrating the exemplary structure of a photo-photo conversion element PPC. In FIG. 2, reference numerals 1 and 2 represent glass plates, 3 and 4 transparent electrodes, 5 and 6 terminals, 7 a photoconductive layer, 8 a dielectric mirror, 9 an optical member (e.g., a photo-modulation layer such as lithium niobate, or nematic liquid crystal layer) which changes the light characteristics with the intensity distribution of an electric field applied thereto, WL a writing light, RL a reading light, and EL an erasing light.
In FIG. 2, the incident direction of the erasing light EL is shown same as that of the reading light RL. Such an incident direction of the erasing light EL is used for the case where the dielectric mirror 8 of the photo-photo conversion element PPC has the light transmittance characteristics that the reading light RL is reflected and the erasing light EL is transmitted.
In writing optical information in the photo-photo conversion element PPC shown in FIG. 2, a circuit made of a power source 10 and a switch SW is connected between the terminals 5 and 6 of the photo-photo conversion element PPC. The movable contact of the switch SW is turned to a fixed contact WR upon reception of a switch control signal supplied to an input terminal 11 of the switch SW. Then, a voltage from the power source 10 is applied between the transparent electrodes 3 and 4 to provide an electric field between opposite sides of the photoconductive layer 7. In this condition, a writing light WL is applied from the glass plate 1 of the photo-photo conversion element PPC to write optical information therein.
Specifically, when the writing light WL incident to the photo-photo conversion element PPC transmits the glass plate 1 and transparent electrode 3 and reaches the photoconductive layer 7, the resistance value of the photoconductive layer 7 changes in accordance with the optical image of the incident light. Therefore, an electric charge image corresponding to the optical image of the incident light is produced at an interface between the photoconductive layer 7 and dielectric mirror 8.
In reproducing from the photo-photo conversion element the optical information written in the form of electric charge image corresponding to the optical image of the incident light, a reading light RL of certain light intensity projected from a light source (not shown) is applied from the glass plate 2 to the photo-photo conversion element PPC while maintaining the movable contact of the switch SW at the fixed contact WR and applying a voltage of the power source 10 via the terminals 5 and 6 between the transparent electrodes 3 and 4.
Specifically, an electric charge image corresponding to the optical image of an incident light is being produced at the interface between the photoconductive layer 7 and dielectric mirror 8. Therefore, an electric field having an intensity distribution corresponding to the optical image of the incident light is being applied to the optical member 9 (e.g., lithium niobate monocrystal) which is in series with the photoconductive layer 7 and dielectric mirror 8.
The refractive index of the lithium niobate monocrystal 9 changes with an electric field applied thereto, because of the electrooptical effect. Therefore, the refractive index of the lithium niobate monocrystal 9 changes with the electric charge image which has changed the electric field to have an intensity distribution corresponding to the electric charge image.
The reading light RL projected toward the glass plate 2 poropagates along the optical path of transparent electrode 4, lithium niobate monocrystal 9, and dielectric mirror 8. The reading light RL is then reflected by the dielectric mirror 8 and returns toward the glass plate 2. In this case, the reading light RL having passed through the lithium niobate monocrystal 9 includes therein image information corresponding to the intensity distribution of the electric field, because the refractive index of the monocrystal 9 has been changed with the electric field. Consequently, a reproduced optical image corresponding to the original optical image appears on the glass plate 2.
In erasing the optical information having been written with a writing light WL, the movable contact of the switch SW is turned to a fixed contact E upon reception of a switch control signal supplied to the input terminal 11 of the switch SW so that the same potential is applied to the terminals 5 and 6 of the photo-photo conversion element PPC without generating an electric field between the transparent electrodes 3 and 4. In this condition an erase light EL having a uniform intensity distribution is applied from the glass plate 2.
According to the image pickup apparatus constructed of the photo-photo conversion element PPC having two transparent electrodes with at least a photoconductive layer, dielectric mirror and photo-modulation layer interposed therebetween, an electric charge image corresponding to the optical image of an object to be image picked up is formed through application of the optical image to the photo-photo conversion element PPC, and the electric charge image is optically read as optical image information which is then photoelectrically converted to obtain video signals with high resolution.
The photo-photo conversion element PPC constituting the image pickup apparatus is, as described above, of a laminated structure made of two transparent electrodes 3 and 4 between which there are provided the components including the photoconductive layer 7, dielectric mirror 8 and optical member 9 (e.g., photo-modulation layer such as lithium niobate monocrystal) which changes the light characteristics in accordance with the intensity distribution of an electric field applied thereto. Each of the components have a static capacitance. Consequently, because of the time constant of the photo-photo conversion element PPC, the electric charge amount of an electric charge image produced at the interface between the photoconductive layer 7 and dielectric mirror 8 increases as the time duration while the writing light is applied becomes long. The electric charge amount may eventually become saturated.
Further, if a new electric charge image is written in the photo-photo conversion element PPC, the old electric charge image on the same frame is first erased, and then the new electric charge image is written. In such a case, in generating time sequential video signals based on an electric charge image, the signal level of each video signal changes with the elapsed time after the erase operation was performed. Thus, a shading occurs in the image reproduced from such video signals.
Furthermore, as described later with FIG. 4, there is an image pickup apparatus of the type that an erase operation is performed during each vertical retrace blanking time, and a read operation is performed during the time other than the vertical retrace blanking time, respectively as shown in FIG. 6(a). In addition, a writing light of the optical image of an object is always applied to the photo-photo conversion element during the write operation. Therefore, time sequential video signals obtained from the photo-photo conversion element PPC have a shading.
Specifically, the electric charge amount of an electric charge image gradually increases during the write operation between two consecutive erase operations. Therefore, the amplitude of each video signal obtained based on such an electric charge image gradually increases.
Shading in video signals caused by the above-described reasons may be eliminated by independently and separately setting the write operation period and read operation period. However, in this case, light exposure time becomes short so that the level of a read-out signal becomes small. Further, video signals without shading can be obtained by setting the read, write and erase operation periods as desired, storing video signals obtained from the electric charge image in a memory such as a franme memory, and thereafter converting them into video signals conforming with a predetermined standard television system. However, this system requires high deflection frequencies and high video signal frequencies in order to allow high resolution of moving images so that the system is difficult to be practiced by using a frame memory presently available.